SvelteKit PWA: Installable App with Offline Access # SvelteKit PWA: Installable App with Offline Access #

🚀 What is a PWA? #

Before we get going on seeing how to build a SvelteKit PWA, let’s run it back a little and first look at what a PWA is. A Progressive Web App (PWA)  is an app built to work with HTML, JavaScript, and CSS but with the functionality of a native app. Features include responsiveness and offline availability, as well as the possibility to install the app. You can test a site's PWA capability in Lighthouse  .

For all of those mentioned features to work, we need to add some additional superpowers to our SvelteKit app. These include icons of various sizes and a manifest file which contains the app name, colours and some other properties. As well as those items, we also need to define a service worker  to perform background caching and do the groundwork to make the app available offline.

In this post, we will look at how to use the PWA functionality in the MDsveX starter blog. After that, we will look at what is under the hood, so you can set up an existing project as a PWA. If that all matches what you were looking for, why don't we get cracking?

🧱 Create a SvelteKit PWA using the MDsveX Starter #

I have just added PWA functionality to the MDsveX starter  . All you need to do is customize the website configuration file with the site's name and theme colours, then run a script to generate all the different icon sizes automatically. Here is exactly what you need to do in five simple steps. If you are looking for a more general SvelteKit Tutorial or how to get started with SvelteKit I have a couple of posts you will appreciate.

    
pnpm build
pnpm preview

  

    
85 // 'Content-Security-Policy-Report-Only': csp,
86 'Content-Security-Policy': csp,

  

I hope you find that helpful and see how easy it can be to set up a PWA in SvelteKit. If you are interested, we will have a quick look under the hood next to see what makes the PWA work.

🔧 What’s under the Hood? #

There are a few elements needed to create a SvelteKit PWA successfully. In this section, we look at them one, by one.

Manifest and Icons #

The manifest.webmanifest used by the PWA is created in the server endpoint file src/routes/mafifest.webmanifest/+server.js. This file creates a static manifest (export const prerender = true in line 3 takes care of this):

    
import website from '$lib/config/website';
export const prerender = true;
export const GET = function get({ setHeaders }) {
	const { backgroundColor, description, siteShortTitle, siteTitle, themeColor } = website;
	const manifest = {
		name: siteTitle,
		short_name: siteShortTitle,
		description,
		start_url: '/',
		background_color: backgroundColor,
		theme_color: themeColor,
		display: 'standalone',
		icons: [
			{ src: '/icon-192.png', type: 'image/png', sizes: '192x192' },
			{ src: '/icon-512.png', type: 'image/png', sizes: '512x512' },
		],
	};
	setHeaders({
		'content-type': 'application/json',
	});
	return new Response(JSON.stringify(manifest));
};

  

HTML head #

There are two elements which you need to add to your page HTML head section on all pages. First, is the theme colour which the browser uses to change the address bar to match your site's colour theme  . Finally, there is an icon which iOS uses as the Home Screen icon when the user installs your app:

    
1 <script>
2   import website from '$lib/config/website';
3   const { themeColor } = website;
4 </script>
5  
6 <svelte:head>
7   <meta name="theme-color" content={themeColor} />
8 		<link rel="apple-touch-icon" href="%sveltekit.assets%/apple-touch-icon.png" />
9 </svelte:head>

  

This component is included in page layout templates. Because we wrap these elements in a <svelte:head> element, SvelteKit works them in the HTML head section of our page.

Service Worker #

The final element is the service worker, which does all the heavy lifting for us in terms of caching data needed for the app to be available offline.

    
1 // based on https://github.com/tretapey/svelte-pwa/blob/master/public/service-worker.js
2 import { build, files, version } from '$service-worker';
3 
4 const worker = self;
5 const CACHE_NAME = `static-cache-${version}`;
6 
7 const to_cache = build.concat(files);
8 
9 worker.addEventListener('install', (event) => {
10 	console.log('[ServiceWorker] Install');
11 
12 	event.waitUntil(
13 		caches.open(CACHE_NAME).then((cache) => {
14 			console.log('[ServiceWorker] Pre-caching offline page');
15 			return cache.addAll(to_cache).then(() => {
16 				worker.skipWaiting();
17 			});
18 		}),
19 	);
20 });
21 
22 worker.addEventListener('activate', (event) => {
23 	console.log('[ServiceWorker] Activate');
24 	// Remove previous cached data from disk
25 	event.waitUntil(
26 		caches.keys().then(async (keys) =>
27 			Promise.all(
28 				keys.map((key) => {
29 					if (key !== CACHE_NAME) {
30 						console.log('[ServiceWorker] Removing old cache', key);
31 						return caches.delete(key);
32 					}
33 				}),
34 			),
35 		),
36 	);
37 	worker.clients.claim();
38 });
39 
40 self.addEventListener('fetch', (event) => {
41 	console.log('[ServiceWorker] Fetch', event.request.url);
42 	if (event.request.mode !== 'navigate') {
43 		return;
44 	}
45 	event.respondWith(
46 		fetch(event.request).catch(() => {
47 			return caches.open(CACHE_NAME).then((cache) => {
48 				return cache.match('offline.html');
49 			});
50 		}),
51 	);
52 });

  

Note in line 5 we use the build version in our cache name. This helps us identify when the cached data is stale. Let me know if you would like more detailed explanations on anything here. I based this code on Pedro Sonzini Astudillo's  Svelte service worker.

🙌🏽 Open Graph SEO in SvelteKit: Wrap Up #

That's it for this post. We have seen:

• what a PWA is,
• how to create a SvelteKit PWA using the MDsveX starter,
• what's under the hood in the starter's PWA code.

As always, suggestions for improvements, together with requests for explanations and feedback, are more than welcome. Also, let me know what other features you would like implemented on the starter.

🙏🏽 SvelteKit PWA: Feedback #

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